The invention relates generally to the field of level instrumentation and measurement. More specifically, embodiments of the invention relate to systems and methods for measuring the total capacity of a plurality of storage tanks sharing a common discharge header.
Many different ways exist to measure and indicate the level or volume of a storage tank. The instrumentation chosen is typically dependent upon the type of material being stored.
If the material is a bulk solid, such as a grain or coal, a vessel or container can be weighed using mechanical scales or an electronic strain gauge, subtracting the tare and converting the known weight to volume. A level instrument relying upon ultrasonics can be employed to gauge the height of the material thereby yielding level and concomitantly volume. Other schemes in the art exist as well.
The two described methods can be applied to liquids in storage tanks. Measuring liquids allows other instrumentation methods to be used such as level transmitters using floats and displacers, and differential pressure transmitters measuring hydrostatic head. All process instrumentation transmitters usually output a signal proportional to the measured parameter when coupled to an instrument loop for local and remote indication and process control.
Measuring the capacity of two or more storage tanks when piped together with a common discharge header can be accomplished using one level instrument, but under careful consideration. Good engineering practice requires separate instrumentation for each tank in light of discharge block valves that can be closed if a tank is removed from service, or if the type of level instrumentation used can lead to operator error. For example, shown in FIG. 1 are two storage tanks coupled together located at different elevations sharing a common discharge. If one float type level transmitter with remote indication is employed on the bottom tank, a full supply will be indicated even when the upper tank is emptied.
The inventors have discovered that it would be desirable to have systems and methods to properly measure the overall capacity of a plurality of storage tanks.
One aspect of the invention provides a method of measuring total capacity for a plurality of storage tanks. Methods according to this aspect of the invention start with deriving a weighting factor for each one of the plurality of storage tanks, determining an overall transmitter resistance for the total capacity measurement, and multiplying each one of the storage tank weighting factors with the overall resistance yielding a span resistance corresponding to each one of the plurality of storage tanks.
The method further includes coupling a transmitter mounted on each one of the plurality of storage tanks in a series loop whereby each transmitter has the span resistance corresponding to the tank mounted thereon and deriving a weighting factor includes adding the capacities of all of the plurality of storage tanks as a sum total capacity and dividing each storage tank capacity from the plurality of storage tanks by the sum total capacity.
The system includes a transmitter mounted on each one of the plurality of storage tanks for measuring a respective storage tank capacity, an overall span resistance, a corresponding weighting factor applied to each transmitter to determine a transmitter span resistance, and a power supply and indicator coupled in series with each of the transmitters whereby total storage tank capacity is measured using each of the transmitter span resistances wherein all of the span resistances equal the overall span resistance. Each of the corresponding weighting factors is determined by a respective storage tank capacity divided by a sum total of all of the plurality of storage tank capacities.
Other objects and advantages of the systems and methods will become apparent to those skilled in the art after reading the detailed description of the preferred embodiments.